1. Field of the Invention
The present invention relates to a low coherent interference fringe analysis method for measuring and analyzing phase information such as surface forms and internal refractive index distributions of a sample by using low coherent light (e.g., white light) including a plurality of light waves having respective wavelengths different from each other as illumination light in an interferometer apparatus.
2. Description of the Prior Art
While low coherent interference fringe analysis method using interference of low coherent light such as white light had conventionally been utilized for measuring surface forms of precision instruments such as lenses and IC boards, their field of application has recently been expanding, for example, so as to be employed in tomographic analyses of eyeballs.
A low coherent interference fringe analysis method comprising the steps of dividing low coherent light as illumination light in a Michelson type interferometer apparatus, for example, into two; irradiating a reference surface with one of thus obtained two light components so as to form reference light; irradiating a sample with the other so as to form object light carrying phase information of the sample; causing the object light and the reference light interfere with each other so as to form interference fringes; and capturing the interference fringes with a CCD camera or the like. When the light intensity of a predetermined pixel of the CCD camera is measured while shifting the sample or reference surface along the optical axis (which may be referred to as “z direction” in the following) of the interferometer apparatus, a light intensity distribution (interference waveform) along the z direction is obtained.
Since the low coherent light includes a plurality of light waves having respective wavelengths different from one another little by little, the interference fringes obtained by the low coherent light are considered to be composed of a plurality of sine functions having respective periods different from one another little by little. Therefore, in the light intensity distribution, the light intensity is maximized at a position where the phase of object light becomes identical to that of reference light at all the wavelengths (i.e., at a position where their optical path lengths are identical to each other) since their light waves enhance each other at all the wavelengths, whereas the light intensity gradually decays as farther distanced from the position.
It is important for the low coherent interference fringe analysis method to determine the position where the light intensity distribution is maximized. Actually obtained light intensity distribution data are discrete, whereas various methods for determining the maximum light intensity distribution position from the discrete data have been proposed.
Examples of known methods include those subjecting a number of discrete data items to Fourier transform or Hilbert transform, or integral transform such as wavelet transform, and those passing the squared AC component of the light intensity distribution to a low-pass filter.
However, such conventional methods require sample point intervals to be sufficiently small, which necessitates an enormous amount of arithmetic operations, thereby lowering the measurement speed.